1. Field of the Invention
The present invention relates to polymer material used for composing optical communication parts, optical waveguides using such polymer materials and a method of fabricating the optical waveguides.
2. Description of the Background Art
In the field of telecommunications, development of optical waveguide has been recognized as a critical issue to enable large capacity communications.
Prerequisites to the materials used for composing optical communication parts such as an optical waveguide include higher transparency at wavelengths in the near-infrared range to which the wavelength of optical signals belongs and less scattering. The materials are also required to have controllability in their refractive indices since they are used to compose optical transmission paths.
Glass or other inorganic crystalline materials have conventionally been used as materials for composing optical communication parts such as an optical waveguide. These materials, however, suffer from their expensiveness and difficulty in processing.
In recent years, polymer materials, such as PMMA (polymethyl methacrylate) and PS (polystyrene), became more popular thanks to their inexpensiveness and easier processing as compared with those of glass or other inorganic crystalline materials. Use of such material can provide a film-type optical waveguide with wider area and higher flexibility than the conventionals. It becomes also possible to obtain a functional optical waveguide by introducing functional compounds or functional groups into such polymer materials.
Fabricating such an optical waveguide essentially requires a method of processing the polymer materials into a desired form. Typical of such a method has been the reactive ion etching (RIE) method using oxygen plasma. The fabrication process of a polymer-made optical waveguide by the RIE method has to be proceeded as generally shown in FIGS. 5A to 5E. Here, FIGS. 5A to 5E show schematic cross-sectional views useful for understanding the major steps in sequence for fabricating a polymer-made optical waveguide using the RIE method.
First, on a base 101, a polymer film 103a as an underclad, a polymer film 103b for forming a core, and a photoresist film 105 for forming an etching mask are formed in this order, FIG. 5A.
To obtain the etching mask corresponded to a desired patterned shape by processing the photoresist film 105, the photoresist film 105 is then subjected to selective light exposure through a photomask 107, FIG. 5B, corresponding to the patterned shape. This results in forming a latent image of the pattern in the photoresist film 105. The photoresist film 105 after exposed with the light is then developed to obtain a resist pattern 105x, FIG. 5C. The example shown here relates to a case with negative photoresist.
RIE with an oxygen-base etching gas is then carried out using the resist pattern 105x as an etching mask 105x, and a portion of the polymer film 103b being exposed from the etching mask 105x is removed. A core 103x made of the residual portion of the polymer film 103b is thus formed on the underclad 103a, FIG. 5D.
On the specimen on which the core 103x has been formed, a polymer film 111 for forming overclad is formed to obtain an optical waveguide 113, FIG. 5E. The overclad 111 can be formed by, for example, coating on the specimen a coating fluid containing material of the overclad, and is then allowed to dry.
As for PMMA and PS, some approaches have been taken to improve transparency in the near-infrared region. More specifically, these materials show absorption ascribable to Cxe2x80x94H bonds in their molecules in the near-infrared region, and thus deuterated PMMA, that is, PMMA whose hydrogen atoms are substituted with deuterium atoms has been developed. Deuterated PMMA shows absorption in the far-infrared region as shifted from the near-infrared region.
The above-described PMMA, PS and deuterated PMMA composing the core of the optical waveguide, however, are low in glass transition temperature. For instance, both of the PMMA and deuterated PMMA have a glass transition temperature of 107xc2x0 C., so that these materials may easily be softened due to heat treatment such as soldering, if they are used to compose electronic parts for computers or so.
These materials also suffer from relatively high water absorption. Both of the PMMA and deuterated PMMA have a value of water absorption as high as 2.0%. The materials composing optical communication parts may alter their refractive indices due to water absorption, which may cause undesirable transmission error in optical communications.
The PS further has a specific problem on birefringence. In the conventional fabrication process of optical waveguides based on the RIE method, a number of steps and a long process time are necessary for forming the pattern, as is clear from the description referring to FIGS. 5A to 5E. Problems also reside in that apparatus used for the RIE method costs high and requires special skills in the operation.
It is therefore an object of the present invention to provide polymer material superior in transparency and free from scattering in the operating wavelength region.
It is another object of the present invention to provide an optical waveguide having a core made of material with higher heat resistance, lower water absorption and no birefringence.
It is still another object of the present invention to provide an optical waveguide simpler in structure and easier in fabrication process without using the RIE method.
The inventors of the present invention, after extensive studies, focused on the fact that imidated polymer material becomes higher in glass transition temperature and lower in water absorption as compared with those of the original material before imidation, which has led to the present invention.
A polymer material according to the present invention contains a repetitive unit having formula (1): 
The material expressed by the formula (1) can be obtained by, for example, reacting deuterated PMMA having superior transparency in the near-infrared region with deuterated methylamine, where deuterated methylamine reacts with the ester bond portion of deuterated PMMA to effect intramolecular imidation. The resultant imide has a cyclic structure. The imidated deuterated PMMA, i.e. deuterated polymethyl methacrylimide, has a higher glass transition temperature and a lower water absorption as compared with those of deuterated PMMA, and has a transparency equivalent with that of deuterated PMMA. The material is thus favorable as the one for optical communication parts.
The present invention also claims a polymer material containing a repetitive unit having formula (2): 
The material expressed by the formula (2) can be obtained by, for example, reacting deuterated PMMA with ethylenediamine, where etylenediamine reacts with the ester bond portion of deuterated PMMA. One repetitive unit and one amino group react each other. Since one ethylenediamine molecule has two amino groups, two repetitive units of deuterated PMMA and one ethylenediamine molecule can react. Thus the products (polymer material) of the reaction will have a structure in which two repetitive units of deuterated PMMA are crosslinked with ethylenediamine. The repetitive unit of this polymer material has two cyclic portions each of which being similar to the above-described deuterated polymethyl methacrylimide. Also this polymer material has a higher glass transition temperature and a lower water absorption as compared with those of deuterated PMMA, and has a transparency equivalent with that of deuterated PMMA. The material is thus favorable as the one for optical communication parts.
An optical waveguide of the present invention comprises a clad and a core, and the core is made of polymer material containing a repetitive unit having formula (1), (2) or (3): 
A polymer material, e.g. polydimethyl glutarimide (PMGI) expressed by the formula (3), can be obtained by, for example, reacting PMMA with methylamine, where methylamine reacts with the ester bond portion of PMMA to effect intramolecular imidation. The resultant imide has a cyclic structure. The imidated PMMA, i.e. PMGI, has a higher glass transition temperature and a lower water absorption as compared with those of PMMA, and shows neither light absorption nor scattering in the operating wavelength region. There is no problem on birefringence. The material is thus favorable as the one for forming the core.
Molecular weights of the polymer materials expressed with formulae (1), (2) and (3) can be selected to arbitrary values according to objects of their use. If these polymer materials to be formed into film by the spin coating method, it is preferable to adjust their degrees of polymerization so that their molecular weights fall within a range from 12,500 to 540,000, and more preferably from 150,000 to 540,000. The viscosity of coating fluid to be coated depends on the evaporation rate of solvent used to dissolve these polymer materials. The viscosity of the coating fluid is thus adjusted based on the molecular weights of these polymer materials and their amounts to be dissolved in the solvent. More specifically, it is preferable to adjust the viscosity of the coating fluid between 100 cP and 10,000 cP. The molecular weights of these polymer materials between 150,000 and 540,000 will facilitate the adjustment of the viscosity of the coating fluid, and polymer film with smooth surface will be obtained.
An optical waveguide with a core fabricated using these polymer materials is high in heat resistance and low in water absorption. Thus using the optical waveguide will successfully provide optical communication parts with an advanced durability against the environment.
In the optical waveguide of the present invention, the clad is preferably made of a material having a smaller refractive index than those of the above polymer materials. PMMA, for example, is recommendable as a material for the clad.
According to another constitution of the optical waveguide of the present invention, the optical waveguide comprises a substrate made of inorganic material; a core formed on the substrate; and an overclad made of polymer and formed on the substrate to cover the core; refractive indices of the substrate and the overclad being approximately the same, and the ratio (n1xe2x88x92n2)/n1 of the difference of the refractive index n1 of the core from that n2 of the substrate or overclad to the refractive index n1 of the core being within a range from 0.3 to 3.0%.
This allows the substrate to be used as an underclad (lower clad) in the conventional sense and provides an optical waveguide with a simpler structure. Since the ratio of the difference in refractive index between the core and the substrate or overclad to the refractive index of the core falls within a range from 0.3 to 3.0%, light used for telecommunications can successfully be transmitted over the optical waveguide based on total reflection.
Glass, being widely used as a substrate material, is recommendable as an inorganic material composing the substrate of the optical waveguide. Polymer composing the overclad can be of a refractive index which matches well with that of the substrate, and is exemplified as UV (ultra violet) curing or setting resin, thermosetting resin and so forth.
The inorganic material of the substrate and the polymer of the overclad are selected so that their refractive indices approximately coincide with each other. When barium borosilicate glass is used as the inorganic material, for example, an UV setting resin (manufactured by NTT Advanced Technology Corporation, product code No. 8101) is typically used as a polymer since it has a refractive index substantially equal to 1.528.
A typical optical waveguide comprises a substrate made of barium borosilicate glass; a core made of PMGI and formed on the substrate; and an overclad made of UV setting resin and formed on the substrate so as to cover the core; thus allowing the ratio of the difference in refractive index between the core and the substrate or overclad to the refractive index of the core to fall within a range from 0.3 to 3.0%.
According to the method of fabricating an optical waveguide of the present invention, the optical waveguide comprising a core on a base is formed by the steps of: forming on the base a core-forming material layer with a positive resist property; subjecting the core-forming material layer to light exposure through a photomask which shadows a core-formative area of the core-forming material layer; and, developing the core-forming material layer after exposed by dipping it into developing solution to leave a portion of the core-forming material layer corresponding to the core-formative area.
By contrast to the conventional process of forming a core on a base, in which a core-forming material layer is formed by coating on a base, a resist pattern is then formed by photolithography, and the core-forming material layer is etched by the RIE method using the resist pattern as an etching mask, the present invention allows core forming only by the steps of forming the core-forming material layer on the base and of effecting light exposure and development process to the core-forming material layer. Thus an optical waveguide can be fabricated quite easily with a lesser number of process steps without using the RIE method.
It is preferable to use, as a core-forming material with a positive photoresist property, PMGI containing a repetitive unit expressed by formula (3). This material is known to cleave at three bonds within a single repetitive unit when irradiated by energy beam with a wavelength of 300 nm or shorter (deep UV light, for example). Formula (4) simply expresses this reaction. 
The light exposed area of the core-forming material layer becomes soluble to developing solution due to the bond cleavage as defined by formula (4).
When developed, the exposed area dissolves to the developing solution to be removed, while the core-formative portion remains. Thus a core can be fabricated in a manner quite easier than in the conventional process.
A base in a context of the present invention typically refers to an underclad (lower clad) formed on an arbitrary base member, and a substrate (including that in a form of film) available as an underclad.
The above-described arbitrary base member for forming the underclad refers to an base member arbitrarily selected depending on a design of the optical waveguide, and either a base member made of inorganic or organic material is available. More specifically, it can be selected from a semiconductor substrate made of silicon or compound semiconductor; a glass substrate; a ceramic substrate; and base material made of arbitrary polymer material other than that used for the underclad. The arbitrary base member can, of course, be an intermediate product in which other electronic parts or optical parts are already incorporated.
A substrate available as an underclad is, for example, made of a material having a refractive index approximately the same with that of the overclad. A substrate made or glass or other inorganic materials is typically available.
A polymer material as a core-forming material preferably has a molecular weight within a range from 12,500 to 540,000.
When the spin coating method is adopted to form the core-forming material layer on the base, the viscosity of a coating fluid to be coated on the base will preferably be within a range from 100 cP to 10,000 cP. Using the coating fluid having a viscosity within the above range allows a desired thickness of a layer to be formed. It becomes also possible to smoothen the surface of the layer formed by the spin coating method. The viscosity of the coating fluid is adjusted based on the molecular weight of the polymer material and the amount of the polymer material to be dissolved in the solvent. It is thus preferable to select the molecular weight of the polymer material between 150,000 and 540,000 to adjust the viscosity of the coating fluid within the above range. The molecular weight is more preferably selected within a range from 150,000 to 540,000.